Turtle just finished his breakfast (clover, dandelion and spring mix) and then, on purpose as he eats from a dish, took a mouthful of sand and ate it. I have play sand and aspen for his substrate and this is the first time I've seen him do that. He wasn't eating right when I got him and it took him a while to eat more than romaine but he is finally eating much better now but I'm guessing something is still missing in his system? What would cause him to do that?